Over the past decade, California farmers have been seeing symptoms of climate change in their fields and orchards: less winter chill, crops blooming earlier, more heat waves and years of drought when the state baked in record temperatures.

Scientists say California agriculture will face much bigger and more severe impacts due to climate change in the coming decades. In a new study, University of California researchers said those effects range from lower crop yields to warming that will render parts of the state unsuitable for the crops that are grown there today.

Two-thirds of the nation’s fruits and nuts are produced in California, along with more than a third of the country’s vegetables. The team of researchers said the warming climate is projected to hit many of those crops hard and will require localized efforts to help growers adapt and prepare for risks.

“The urgency of addressing these issues has become critically important,” the scientists said in the study.

Their review of scientific research examined climate trends and the current and future effects on farming in California. They consulted 89 research papers and reports, and presented a list detailing what studies have found about impacts on agriculture.

Among the key findings, more than half of the Central Valley is projected to be no longer suitable for growing crops like apricots, peaches, plums and walnuts sometime around the middle of the century. By the end of the century, that’s projected to grow to 90 percent or more of the valley.

A worker carries a box of peaches while picking fruit in an orchard near Kingsburg, California, in 2015. (Photo: Ian James/The Desert Sun)

Crop yields are expected to fall for crops including almonds, table grapes and cherries, among others.

Summer heat waves are likely to take a toll on corn, rice, tomato and sunflower crops.

Crops may also be plagued by pests, diseases and weeds that can flourish in warmer temperatures.

“One under-appreciated aspect of California’s climate is how important our temperature envelope is to California’s agricultural sector. The right temperatures at the right times are absolutely crucial,” said Faith Kearns, a scientist with the California Institute for Water Resources who was part of the team. “For example, warm weather in January and February can reduce almond yields, but warm summers can reduce peach yields. So, there really is no-one-size-fits-all adaptation approach.”

As human-caused climate change brings bigger rises in average temperatures, decreases snowpack and changes precipitation patterns, those changes will have largely negative efforts for California’s agricultural sector, Kearns said, and will play out in the timing of flowering, the number of chill hours and the water needs of the crops, among other things.

More frequent and more intense weather extremes, from droughts and heat waves to flooding, are also signatures of climate change that will have major effects on agriculture and could significantly harm the state’s ag economy, said Tapan Pathak, a scientist and climate adaptation extension specialist at the University of California, Merced, who was the lead author.

“There is a clear need and urgency for adaptation research to make California agriculture resilient to future climate risks,” Pathak said. In that localized research, he said, “priority should be given to crops and commodities that are most vulnerable to climate impacts.”

California grows more than 400 different crops, and some of them are only cultivated here.

Water flows into an almond orchard east of Fresno. Many farms in California’s Central Valley still rely on flood irrigation, and groundwater levels have been declining for decades. Growing numbers of farms have been switching to drip irrigation. (Photo: Jay Calderon/The Desert Sun)

Because California agriculture is diverse and all crops respond differently to changes in climate, Kearns said adaptation efforts will need to be specific and localized.

“In some cases that will mean further research, coupled with working closely with growers to address specific needs,” Kearns said.

The study was published on Monday in the journal Agronomy. The researchers said impacts on agricultural output “could disrupt state and national commodity systems.”

“While California farmers and ranchers have always been affected by the natural variability of weather from year to year, the increased rate and scale of climate change is beyond the realm of experience for the agricultural community,” the scientists said.

Nuts and other permanent crops have been profitable for California growers lately, and new almond and pistachio orchards have been planted across the Central Valley in recent years, adding to strains on the declining groundwater aquifers.

But many fruit and nut trees require cold temperatures in winter. The study said pistachios, for example, require temperatures between freezing and 45 degrees for about 700 hours each winter, but for the past four years there have been less than 500 chill hours.

The temperatures that climate models project for the end of this century would no longer support some of California’s main tree crops, the researchers said. Some of the most climate-sensitive trees are walnuts, which require the highest number of chill hours, and that points to a decline of walnut orchards in many parts of the state.

For now, about 99 percent of the country’s walnuts are grown in the Central Valley.

Some crops are expected to fare better than others. Studies have projected very small changes in the yields of wine grapes. Alfalfa is another crop that based on research to date isn’t expected to see much of an impact due to climate change, at least as long as water is available to irrigate the crop.

But substantial declines are expected due to climate change for a number of other crops, including table grapes, almonds and avocados.

“I don’t see a lot of winners, and I see a lot of big losers,” said Peter Gleick, a water and climate expert who is president emeritus of the Oakland-based Pacific Institute. “There are some crops here that maybe do OK, like wine grapes, but they’re the exception.”

“What it means is we can’t ignore climate change in our agricultural planning,” said Gleick, who wasn’t involved in the study. “The growers that are planting crops today, especially permanent crops, can’t assume that tomorrow’s climate is going to look like today’s climate, because it isn’t. They can’t assume that they’re going to get the yields they expect based on past performance.”

He said individual farmers will need to look at specifics of their local climate and the water they have available. But heat stress and the decrease in chill hours will be issues everywhere.

The researchers also considered different scenarios of climate change, including a scenario in which people take significant actions to reduce emissions of greenhouse gases, and a scenario in which people continue with emissions at current levels.

Crops including tomatoes, rice, cotton and sunflowers would far significantly better under the low-emissions scenario.

“But the business-as-usual scenario, the path that we’re actually on, is much more devastating,” Gleick said. “So the scenario matters, the severity of the problem matters, which means what we do today matters, not just in agriculture but on the emissions side of the equation.”

In the study, the scientists said for crops that are sensitive to extreme heat, research to breed and test more heat-tolerate varieties is a high priority. They said other research efforts should focus on localized and crop-specific adaptation efforts, and on finding irrigation methods that save water while minimizing losses in crop yields.

The researchers pointed to the depletion of groundwater and decreases in mountain snowpack as trends that are adding to the strains, saying a decrease in water availability could reduce the areas where crops are grown.

Don Cameron, who manages the farm Terranova Ranch southwest of Fresno, said his operation has been grappling with the effects of climate change, including more wild weather swings. Last winter, California was soaked with record rainfall, and this winter has been one of the driest on record.

Last summer, a heat wave damaged tomatoes and carrots on his farm.

“Our crops really took a beating in yield and quality,” Cameron said. “Our crop was probably down 20 percent.”

The farm’s almond trees bloomed early this winter due to the warmth. Then a freeze damaged some of bloom and the little nuts, and cold temperatures have inhibited bees from coming out to pollinate the trees, Cameron said.

“The almond crop is in jeopardy,” he said. “We’re seeing more extremes than we had before.”

Workers on his farm have started planting peppers later in the summer to avoid the damaging heat. Cameron said the scorching summer heat and warmth in the fall present a growing challenge — and one that could prompt growers to change crops.

“If our yields become affected to the point where we can’t be sustainable, then we’ll change how things are grown here,” Cameron said. “We’re pretty innovative when it comes to finding solutions.”